Field of the Invention
The present invention relates to a current mirror circuit which can be operated at a low voltage. FIG. 1 is a circuit diagram showing a conventional current mirror circuit constituted of PNP transistors. In FIG. 1, reference numeral 71 denotes an input transistor whose base-collector path is short-circuited, and reference numerals 72 and 73 indicate output transistors the bases of which are connected to that of transistor 71. The collector of transistor 71 is connected to input current source 74, and the collectors of transistors 72 and 73 each output current Iout which is proportionate to a current flowing into input current source 74.
In the conventional current mirror circuit, a voltage obtained by subtracting voltage VBE between the base and emitter of transistor 71 from power supply voltage Vcc is applied to input current source 74 as an operating voltage. Let us consider that a low voltage of 0.9 V is applied to the input current source as power supply voltage Vcc. If voltage VBE between the base and emitter of transistor 71 is 0.7 V, the operating voltage applied to input current source 74, has a low value of 0.2 V. The current mirror is thus limited in its application field. This problem will be explained, referring to a differential amplifier circuit illustrated in FIG. 2.
FIG. 2 is a circuit diagram showing a differential amplifier circuit which can be operated at a low voltage. The differential amplifier circuit includes two conventional current mirror circuits, as shown in FIG. 1. One of the current mirror circuits is indicated by reference numeral 81 and constituted of transistors 82 and 83, and the other current mirror circuit is represented by numeral 84 and constituted of transistors 85 and 86. While the current mirror circuit shown in FIG. 1 comprises two output transistors, the current mirror circuits shown in FIG. 2 each comprises a single output transistor. An input current source (corresponding to input current source 74 in the circuit shown in FIG. 1) which supplies an input current to current mirror circuits 81 and 84, consists of differential amplifier 91 including differential pair 89 of NPN transistors 87 and 88 and constant current source 90 connected to a common emitter of differential pair 89. The collector currents of transistors 87 and 88 included in differential amplifier 91 which is supplied with differential input signal Vin, are supplied to current mirror circuits 81 and 84, respectively.
Let us think about a case where the differential amplifier circuit shown in FIG. 2 is operated at a power voltage of 0.9 V. If voltage VBE between the base and emitter of transistor 82 or 85 is 0.7 V, differential amplifier 91 is applied with a voltage of only 0.2 V. Since a saturation voltage (V CEsat) of 0.15 V needs to be secured as a voltage between the collector and emitter of transistor 87 or 88, the operating voltage of constant current source 90 is 0.05 V at the most and, in other words, there is no operating voltage enough to operate constant current source 90. Though constant current source 90 may be constituted of a resistor, its resistance must be sufficiently lowered in order to ensure a considerably large current. Some problems, such as a problem wherein the gain of differential pair 89 decreases and the bias current varies according to the base voltage bias condition of differential pair 89, will then arise. Constant current source 90 can thus be constituted of a transistor whose emitter is grounded, in which case, the lowest voltage Vccmin of power supply voltage Vcc is given by the following equation: EQU Vccmin =VBE +2 VCEsat =0.7 +2 .times.0.15 =1 (V).
As is apparent from the equation, the constant current source does not operate when power supply voltage Vcc is lowered below 1 V.
As mentioned above, the conventional current mirror circuit has the drawback wherein the base-collector path of the input transistor is short-circuited so that a voltage supplied to the input current source is lowered and thus the application field of the current mirror circuit is limited.